1. Field of the Technology
The present technology relates to a fixing device for fixing a toner image and an image forming apparatus including the fixing device.
2. Description of the Related Art
An electrophotographic image forming apparatus for forming an image based on electrophotography (hereinafter simply referred to as an “image forming apparatus”) is able to easily form an image having favorable image quality. The image forming apparatus is widely used as a copier, a printer, a facsimile apparatus, a multi-functional peripheral, and the like.
Examples of a fixing device provided in such an image forming apparatus include a fixing device of heat roller fixing type.
The fixing device of heat roller fixing type includes a fixing roller and a pressure roller. The fixing roller and the pressure roller are a pair of rollers in pressure-contact with each other. Inside at least one of the fixing roller and the pressuring roller, a heat source such as a halogen heater is included as a heating section.
In the fixing device, after the pair of rollers is heated with the heat source to a predetermined temperature required for fixing (hereinafter referred to as a “fixing temperature”), recording paper such as paper on which an unfixed toner image is formed is supplied to a fixing nip section which is a pressure-contact part of the fixing roller and the pressure roller. The unfixed toner image passing through the fixing nip section is fixed onto the recording paper by heat transferred from at least either one of the fixing roller and the pressure roller as well as pressure by the fixing roller and the pressure roller. The temperature of a part of the fixing nip section through which the recording paper has passed (hereinafter referred to as a “paper passing section”) is decreased but heated to the fixing temperature by the heat source.
When a fixing roller on which surface an elastic layer is provided (hereinafter referred to as an “elastic roller”) is used as the fixing roller, at the fixing nip section, the elastic layer on the surface of the elastic roller is elastically deformed corresponding to unevenness of an unfixed toner image. The elastic roller contacts with the unfixed toner image so as to cover and wrap the unfixed toner image, thus making fixation properties favorable for an unfixed color toner image with a large amount of a toner compared to a monochromatic image. Moreover, the elastic roller improves release properties of a color toner which is easier to offset compared to the monochromatic image by a strain release effect of the elastic layer on the surface of the elastic roller.
A nip shape as a shape between the fixing roller and the pressure roller on the fixing nip section becomes a protruded shape on the fixing roller side (an inverse nip shape), thus improving peeling properties between the fixing roller and recording paper. As a peeling section for peeling recording paper from the fixing roller, for example, self-stripping is realized which is capable of peeling recording paper from the fixing roller without using a peeling claw so as to resolve an image defect caused by the peeling section. In a fixing device included in an image forming apparatus capable of performing full-color printing, for example, silicone rubber or the like is used as an elastic layer of the elastic roller.
In the fixing device included in the image forming apparatus capable of performing full-color printing, it is necessary to widen a nip width as a width in a conveyance direction of recording paper of the fixing nip section to cope with higher speed. As a way of widening the nip width, there are two methods. That is, one is to thicken the elastic layer of the elastic roller, whereas the other is to enlarge a diameter of the elastic layer. However, since the elastic layer of the elastic roller has very low heat conductivity, when the elastic layer of the elastic roller having the heating section inside the elastic roller is thickened, in a case where processing speed is increased, it becomes impossible for a temperature of the fixing roller to follow the fixing temperature. Moreover, when a diameter of the elastic roller is enlarged, warm-up time is extended and power consumption at the heating section is increased.
In order to solve such problems, Japanese Unexamined Patent Publication JP-A 9-218601 (1997) discloses a belt fixing type fixing device including a fixing roller, a pressure roller, a heating roller and a fixing belt. In the fixing device disclosed in JP-A 9-218601, a fixing belt is suspended between a fixing roller and a heating roller having inside of which a heater for heating, and the fixing roller is brought into pressure-contact with the pressure roller through the fixing belt.
In such a fixing device, warm-up time is short since the fixing belt having a small heat capacity is heated, and it is not necessary to incorporate a heat source such as a halogen lamp in the fixing roller, thus a thick elastic layer with low hardness made of sponge rubber or the like is able to be provided, so that it is possible to secure a wide nip width.
However, in the conventional belt fixing type fixing device as disclosed in JP-A 9-218601, there is room for improvement in fixation properties when an unfixed color toner image requiring a large amount of a toner is fixed to recording paper having a surface shape with significant unevenness.